Electrical appliances and devices differ widely with respect to the amounts of electricity they consume, which, in turn determines the cost of running the appliances. However, in most cases, the amount of electricity consumed by an appliance is not readily apparent. Many devices are equipped with an information tag which states the power consumption, but this is typically an average or maximum, and does not allow straightforward monitoring of most devices such as solid state TVs which often draw power even when "off". Electrical power consumed by loads which vary such as breadmakers, microwave ovens, dishwashers, compressors, etc, are even more difficult to monitor accurately. In addition, many common appliances utilize reactive elements, such as motors and transformers, which further complicate power measurement since power factor must be taken into account.
It is accordingly an object of the present invention to provide a power meter which readily and accurately provides data regarding the real instantaneous power and consumed power by an electrical device.
The accurate measurement of real power is possible using expensive lab and commercial use type instruments, but these are not very economical or typically designed for easy use by an untrained individual. Several patents have been granted for devices similar to the present invention, however, they share several significant shortcomings. U.S. Pat. Nos. 4,401,943 to Morris (1983) and 5,315,236 to Lee (1994) both disclose devices designed to easily monitor power consumption by electrical devices, however, neither device corrects for power factor, instead reading apparent power. Using either device to monitor the power consumption of a common device such as an air conditioner or refrigerator would present inaccurate results. Neither device displays both instantaneous and consumed power. The device disclosed by Morris is very simple and easy to use, however the display is analog meter which would be hard to accurately interpret over a wide range of values. Lee's device uses a digital display, however, current is sensed by an inductive coil which is expensive, if accurate and sensitive to a wide range of currents. An article in Radio-Electronics, December 1991 issue, by Paul Brule discloses a Energy Consumption Monitor which uses resistive current sensing (for accuracy and span) and corrects for power factor. However, Brules device suffers from excessive heatup of the resistors at high loads, relatively inaccurate measurement of inductive loads, and a relatively high sensing threshold. Furthermore, Brules device uses a transformer to increase the voltage induced across the load resistors. This transformer element results in several disadvantages: The transformer is relatively expensive and bulky and is subject to induced noise from the device's power supply transformer when placed in close proximity. In addition, the device requires additional equipment to monitor instantaneous power.
Thus, the prior art includes devices which include one or more of the following disadvantages:
a) Measure only apparent power, leading to large inaccuracy when measuring reactive devices. PA1 b) Do not display both instantaneous and consumed power. PA1 c) Have poor accuracy and high threshold. PA1 d) Use an inductive element to sense current which is not suitable for low cost and high accuracy. PA1 e) Not designed for easy use. PA1 f) Are not economical to manufacture or purchase. PA1 a) to provide a power measuring device which will measure real power; PA1 b) to provide a power measuring device which will display both instantaneous and consumed power; PA1 c) to provide a power measuring device which will have high accuracy and low threshold; PA1 d) to provide a power measuring device which will use resistive elements to sense current; PA1 e) to provide a power measuring device which will is designed for easy use; and PA1 f) to provide a power measuring device which will b economical to produce and market.